ch.6 cast in drilled hole piles - caltrans

7/30/2019 Ch.6 Cast in Drilled Hole Piles - Caltrans

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6C H A P T E R

JULY1997

CALTRANS FOUNDATIONMANUAL 6-1

Cast-In-Drilled-HolePiles

DescriptionFew terms are as self-descriptive as the one given the Cast-In-Drilled-Hole (CIDH) pile. Theyare simply reinforced concrete piles cast in holes drilled to predetermined elevations. Muchexperience has been gained with this pile type because of their extensive use in the con-struction of bridge structures. While they probably are the most economical of all com-monly used piles, their use is generally limited to certain ground conditions.

The ground formation in which the holes for CIDH piles are to be drilled must be of such anature that the drilled holes will retain their shape and will not cave in when concrete is

placed. Because of cave-in and concrete placement difficulties, these piles are not recom-

mended for use as battered piles. Nor are they recommended where groundwater is present,unless dewatering can be done without unreasonable effort and unless concrete can be

placed without a casing having to remain in place. If groundwater or caving conditions arepresent, the piles can be constructed by the slurry displacement method if permitted in thecontract specifications. The slurry displacement method is described in detail in Chapter 9of this manual.


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JULY1997 Cast-In-Drilled-Hole Piles

6-2 CALTRANS FOUNDATIONMANUAL

Specifications

The Standard Specifications describe two different types of CIDH pile. The first type is thecast-in-drilled-hole pile. The second type is the cast-in-driven-steel-shell pile. For this typeof pile, a steel shell is driven to a specified tip elevation and bearing value. The material

within the steel shell is then removed and the steel shell is filled with reinforced or non-reinforced concrete.

The Standard Specifications contain much of the information necessary to administer theconstruction of CIDH piles. Section 49-4 contains information on the construction method

and hole drilling. Section 52 contains information on pile bar reinforcement. Section 90contains information on the concrete mix design, transportation of concrete, and curing ofconcrete used for CIDH piles.

The Special Provisions contain job-specific requirements and revised specifications. Becausethe CIDH pile specifications are continually updated, it is very important that the StructureRepresentative carefully review the Special Provisions and any revised specifications notedshould be discussed with the Contractor.

Drilling EquipmentThe drilling auger is the most commonly used

piece of equipment for drilling holes for CIDHpiles. Augers may be used in granular andcohesive materials.

There are two basic varieties of augersthestandard short section (Figure 6-1) and con-tinuous flight. Both have flights of varyingdiameter and pitch.

Continuous flight augers have flight lengthswhich are longer than the hole to be drilled.They are generally lead-mounted. The powerunit is located at the top of the auger and ittravels down the leads with the auger as thehole is drilled. Drilling is performed in one Figure 6-1: Auger Short

Section


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Cast-In-Drilled-Hole Piles JULY1997


continuous operation. As the auger moves down the hole, the drillingaction of the flights forces the drill cuttings up and out of the hole.Hence, much material has to be shoveled away from around the drilledhole. Continuous flight augers are most commonly used for short pilesor for predrilling for driven piles. They may also be used where overheadclearance is not a problem.

Short flight augers are powered by Kelly Bar units fixed to the drill rig.The length of these augers generally vary between 5 and 8 feet. Theauger is attached to the end of the Kelly Bar and, as drilling progresses,

the auger (and material carried on the flights) must be removedfrequently. After the auger is removed from the drilled hole, the materialis spun off the flights onto a spoil pile and the operation is repeated.

There are also a variety of different types of augers that may be used indifferent situations. Augers may be single flight (Figure 6-2) or double

flight (Figure 6-3). Double flight augers are better balanced than singleflight augers and are more useful when alignment and location of thedrilled hole are important due to clearance or right-of-way problems.

Soil augers are equipped with a cutting edge that cuts into the soil

during rotation. The drill cuttings are carried on the flights as the augeris removed from the drilled hole and are then spun off. The pitch ofthe flights can vary and should be chosen for the type of materialencountered. Soil augers may not work well in cohesionless materials

where the soil will not stay on the flights during auger extraction. Theymay also not work well in highly cohesive materials where the augermay become clogged.

Rock augers are equipped with high-strength steel cutting teeth that cancut through soft rock. These augers typically have flights with a very

shallow pitch so that rock pieces, cobbles and boulders can be extracted.Rock augers are generally the preferred tool for drilling in materialsthat have a high concentration of cobbles or boulders.

Figure 6-2: Auger Single

Flight

Figure 6-3: Auger Double

Flight


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Drilling buckets (Figure 6-4) are used when augersare not able to extract material from a drilled hole.This can happen when wet materials or cohesionlessmaterials are encountered. Drilling buckets may alsobe appropriate when heavy gravel or cobbles areencountered. Drilling buckets have a cutting edge

which forces material into the bucket during rotation.When the drilling bucket is full, the bucket is spun inthe direction opposite of drilling to close the built-in

flaps, which prevent the cuttings from falling out of

the bucket. The bucket is then extracted from thedrilled hole and emptied.

Cleanout buckets are specialized drilling buckets thatare used to clean loose materials from the bottom of adrilled hole and to flatten the bottom of the drilledhole. This allows the tip of the pile to be founded on a

firm flat surface. These buckets have no cutting teethbut are similar to drilling buckets in other aspects.Specialized cleanout buckets can be used to extract

loose materials when groundwater or drilling slurry ispresent. These buckets, referred to as muckoutbuckets, allow fluid to pass through them whileretaining the loose materials from the bottom of thedrilled hole. Figure 6-5 shows the difference betweenthe cleanout bucket and the drilling bucket.

Figure 6-4: Drilling Bucket


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Figure 6-5: Drilling Bucket/Cleanout Bucket Comparison

Figure 6-6: Core Barrel

Core barrels (Figure 6-6) are usedto drill through hard rock forma-tions, very large boulders orconcrete. This type of drilling toolconsists of a steel cylinder withhard metal cutting teeth on thebottom. Rock cores are broken offand extracted from the drilled holeas a single unit, or may be brokenup with a rock breaker and thenextracted with a drilling bucket orclamshell.


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Drilling is performed almost exclusivelywith portable drilling rigs. These units canbe self-propelled (Figure 6-7), truck-mounted (Figure 6-8), or crane-mounted(Figure 6-9).

Figure 6-7: Drill Rig Crawler

Mounted

Figure 6-8: Drill Rig Truck Mounted

Figure 6-9: Drill Rig Crane Mounted


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Steel casings (Figure 6-10) are used to support drilled holes when unstable conditions areencountered. Various methods have been used to advance steel casings, among them,spinning the casing with the Kelly Bar while applying some vertical force, driving thecasing with whatever means are available as the hole is drilled, or using a vibratoryhammer. Steel casings are generally extracted from the hole in the manner specified in thecontract specifications as concrete is placed.

Drilling MethodsVarious other materials are used to supplement the drilling work. Water is sometimes addedto certain ground formations to assist drilling and lifting materials from the hole. Soil maybe placed back into the hole to dry out supersaturated materials. The drilling tool is used toagitate the materials so they can be extracted from the hole. This is known as processingthe hole.

Figure 6-10: Steel Casing


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Drilling Problems

The difficulties encountered in drilling can include cave-ins, groundwater, and utilities.The following briefly describes some actions which can be taken in these situations.

In the case of cave-ins, the following action or combination thereof may be required:

ITEM ACTION

1 Placement of a low cement/sand mix and redrilling the area of the cave-in.2 If permitted by the contract Special Provisions, use of a drilling slurry (refer

to Chapter 9 of this manual).3 Use of a casing which is pulled when placing concrete.

In the case of groundwater, the following action or combination thereof may be required:

ITEM ACTION

1 Placement of a low cement/sand mix and redrilling the hole.2 Drill ing to tip elevation, using a pump to remove the water and cleaning

out the bottom of the pile.3 If permitted by the contract Special Provisions, use of a drilling slurry (refer

to Chapter 9 of this manual).

4 Placement of a casing, again using a pump to remove the water, andpulling casing during concrete placing (keeping bottom of casing belowthe concrete surface).

5 Dewatering the entire area using well points, deep wells, etc. This should bethoroughly discussed with the Bridge Construction Engineer and theEngineering Geologist.

6 By contract change order, substitute an alternative type of piling. Again,this should be discussed with the Project Designer, the EngineeringGeologist, and the Bridge Construction Engineer.

Operations should proceed with caution when drilling near utilities known or thought to be

in close proximity. The Contractor should contact the utility company and have the utilitylocated. It is also advised that the Contractor pothole and physically locate the utility priorto drilling. Relocation of the utility may be required. Minor adjustments in pile locationmight be feasible in order to avoid conflict. Any proposed revisions to the pile layout shouldbe discussed with the Project Designer, the Engineering Geologist, and the Bridge Construc-tion Engineer.


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In the case of problems with groundwater, cave-ins or obstructions at the lower portion ofthe hole and under certain conditions, the Standard Specifications allow the Contractor to

propose increasing the pile diameter in order to raise the pile tip. Before allowing this, theStructure Representative should consult with the Project Designer and Engineering Geolo-gist to see if this is feasible, and if so, to obtain the revised tip elevation. Appropriate pay

provisions are also included in the contract specifications and a change order is notrequired.

Ordinarily, the above problems would stimulate the Contractors action and a change wouldbe proposed to the Structure Representative.

Inspection

Before drilling begins, it is advisable to have a pre-construction meeting with the Contractorand any subcontractors that will be involved in the work. Items to be discussed shouldinclude any recently revised contract specifications, the contract pay limits, the Contractors

planned method of operation, the equipment to be used, the plan for avoiding existingutilities (if any), and safety precautions to be taken during the work.

The Structure Representative should review the contract plans, the Foundation Report andthe Log of Test Borings thoroughly. If there are any discrepancies noted between the piletype shown on the plans, the pile type called for in the Foundation Report, and the soilmaterials and groundwater level shown on the Log of Test Borings, the Project Designershould be contacted for clarification.

Due to recent changes in design practice, it is possible that the pile may not be designed forend bearing alone. The Project Designer should be contacted to determine whether skin

friction has a role in the capacity of the pile. This is especially true of seismic retrofitprojects, where pile tensile capacity may be a requirement.

The Contractor is required to lay out the pile locations at the site prior to drilling. Thislayout should be checked by the Structure Representative prior to drilling. The StructureRepresentative should also set reference elevations in the area so pile lengths and pile cutoffcan be ascertained.

During the drilling operation, the Structure Representative should verify that the piles arebeing drilled in the correct location and are plumb. Usually, the Contractor will check theKelly bar with a carpenters level during the drilling operation. The Structure Representative


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should also evaluate the material encountered and compare it to that shown on the Log ofTest Borings. If the material at specified tip differs from that anticipated, a change may beneeded. It may be advisable to keep a written record of the drilling progress and the recordutilized to investigate any differing site conditions claims submitted by the Contractor.

When the hole has been drilled down to the specified tip elevation, the Contractor shouldalways use a cleanout bucket to remove any loose materials and to produce a firm flatsurface at the bottom of the drilled hole.

After drilling, the depth, diameter and straightness of the drilled hole must be checked. The

drilled hole should be checked using a suitable light, furnished by the Contractor, or amirror. At this time, the Structure Representative should measure and record the length ofeach pile. Unless the Structure Representative orders the Contractor, in writing, to changethe specified tip elevation, no payment will be made for any additional depth of pile belowthe specified tip elevation.

For large diameter piles, it may be necessary for the Structure Representative or the Engi-neering Geologist to inspect the bearing surface at the bottom of the drilled hole. All

pertinent requirements of the Construction Safety Orders shall be met before anyone entersthe drilled hole.

Immediately before placing concrete, the bottom of the drilled hole should be checked forloose materials or water. Loose materials and small amounts of water can be removed witha cleanout bucket. Large amounts of water may need to be pumped out. It may be neces-sary to remove the rebar cage to accomplish this. Steel reinforcement cage clearances andblocking should also be checked at this time. In addition, the reinforcing cage must beadequately supported and some means must be devised to ensure concrete placement to the

proper pile cutoff elevation.

Concrete placement warrants continuous inspection. This subject is covered comprehen-sively in the contract specifications. Those involved in the work should thoroughly review

Standard Specifications Sections 49-4 and the contract Special Provisions. Applicableportions of Section 90 should also be reviewed with respect to concrete mix design, consis-tency of the concrete mix, and concrete curing requirements.


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Pile Defects

The drilling problems mentioned previously, if not corrected, can cause CIDH piles to bedefective. There are also problems that can occur during concrete placement or casingremoval that can cause defective CIDH piles.

The following drilling problems can cause pile defects:

ITEM DRILLING PROBLEM/PILE DEFECT

1 The Contractor does not clean off the bottom of the dril led hole with a

cleanout bucket. This can result in the pile bearing on soft material. ForCIDH piles designed for end bearing, this flaw can seriously compromise thevalue of the pile. This defect is shown in Figure 6-11.

2 The Contractor uses a tapered auger to advance the drilled hole to thespecified tip elevation and does not flatten the bottom of the hole with acleanout bucket. This can result in concrete crushing at the tip of the pile,

which would reduce its capacity and possibly cause differential settlement.There may also be soft material at the tip of the drilled hole, which wouldcause the problems mentioned previously. This defect is also shown inFigure 6-11.

3 The drilling operation smears drill cuttings on the sides of the drilled hole.This can result in the degradation of the piles capacity to transfer loadsthrough skin friction. This may be critical if the pile is designed as a

tension pile. This condition is most likely to occur in ground formationscontainin cohesive materials. This defect is shown in Fi ure 6-12.

These problems are preventable. Adherence to the contract specifications and timelyinspection will eliminate most of these problems.


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Figure 6-11: Pile Defects

No Cleanout, Tapered Bottom of Hole


Smeared Drill Cuttings


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The following concrete placement problems can cause pile defects:

ITEM CONCRETE PLACEMENT PROBLEM/PILE DEFECT

1 A cave-in at a location above the top of concrete or sloughing material fromthe top of the drilled hole occurs during concrete placement. This can resultin de raded concrete at the location, thus reducin the capacit of the pile.This defect is shown in Figure 6-13.

2 The Contractor tailgates concrete into the drilled hole without the use of ahopper or elephant trunk to guide it. The concrete falls on the rebar cage orsupporting bracing and segregates. This can result in defective concrete,thus reducing the capacity of the pile. This defect is shown in Figure 6-14.

3 A new hole is drilled adjacent to a freshly poured pile or concrete is placed ina drilled hole that is too close to an adjacent open drilled hole. This canresult in the sidewall blowout of freshly poured piled into the adjacentdrilled hole. This would probably cause the rebar cage to buckle. This defectis shown in Figure 6-15.

4The Contractor does not remove groundwater from the drilled hole. Thisgroundwater mingles with the concrete placed and may result in defectiveconcrete at the bottom of the pile. If the pile is designed for end bearing, thecapacity would be reduced. This defect is shown in Figure 6-16.

Most of these problems are preventable. Adherence to the contract specifications and timelyinspection will prevent most of these problems. However, if a cave-in occurs during concrete

placement, the Contractor may need to remove the rebar cage and concrete, and then startover.


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Figure 6-13: Pile Defects Figure 6-14: Pile Defects

Cave In Concrete Segregation


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Figure 6-15: Pile Defects Figure 6-16: Pile Defects

Adjacent Hole Blowout Water in the Hole


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The following casing removal problems can cause pile defects:

ITEM CASING REMOVAL PROBLEM/PILE DEFECT

1 The Contractor waits too long to pull the casing during concreteplacement. This may result in three problems: (1) the concrete sets up andcomes up with the casing as shown in Figure 6-17(a), (2) the concrete setscasing cannot be removed as shown in Figure 6-17(b), and (3) the concretesets up enough so that it cannot fill the voids left by the casing as it isremoved, as shown in Figure 6-17(c). The first problem may result in a voidbeing formed in the pile at the bottom of the casing. It is possible that thesuction created may cause a cave-in at this location. The second and third

problems result in the loss of the piles capacity to transfer skin friction tothe ground.

Historically, problems with casings have produced the largest number and the worst type ofCIDH pile defects. However, these problems are preventable. Adherence to the contractspecifications and timely inspection will prevent most of these problems. It is recommendedto allow the penetration value of the concrete placed in the pile to be at the high end of theallowable range. Research has shown that concrete with higher fluidity will consolidate and

fill in the voids better than concrete with lower fluidity. To further prevent CIDH pile defectproblems when casings are used in construction the CIDH pile contract specifications havebeen revised so that all CIDH piles constructed with the use of temporary casings will betested for structural adequacy prior to acceptance. The pile testing methods used to test pilesconstructed by the slurry displacement method (as described in Chapter 9 of this manual)

would be used in this circumstance.


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(a) (b) (c)


Casing Problems


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Safety

As with all construction activities, the Structure Representative should be aware of safetyconsiderations associated with the operation. As a minimum, the Structure Representativeshall review the Construction Safety Orders that pertain to this work. A tailgate safetymeeting should be held to discuss the inherent dangers of performing this work before the

work begins.

The primary and obvious hazard encountered with CIDH pile construction is the opendrilled hole. Common practice is to keep the drilled hole covered with plywood, especially if

the drilled hole is left open overnight. This provides protection not only for the constructioncrew working in the area, but also the public. In urban areas, more stringent measures maybe required to secure the site.

As with any other type of operation, common sense safety practices should be used whenworking around this equipment. If you do not need to be there, stay away from the equip-ment. If a crane-mounted drilling rig is used, the crane certificate should be checked.

In addition, footing excavations should be properly sloped or shored. Imposed loads, suchas those from cranes and transit mix concrete trucks, must be kept a sufficient distance

from the edge of the excavation. If the Contractor intends to place equipment of this type

adjacent to the excavation, the load must be considered in the shoring design and/or indetermining the safe slope for unshored excavations.

ch.6 cast in drilled hole piles - caltrans

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